# Can you use the kinetic energy of air to power a car?

In my fluid mechanics class, I've learned that a fluid traveling in a pipe will generate a force when the exit area is smaller then the entrance area. Suppose a pipe is attached to a car that will use the kinetic energy of the air passing over the vehicle. Assuming that the car is moving at a constant velocity and the pipe is straight and the exit area is 1/4 the size as the frontal area, is it feasible for a moving vehicle to generate enough force from the air to reduce energy use?

• sign up for thermodynamics class next Mar 2 '17 at 5:19
• Here ya go. britishlandsailing.org.uk/class-3.html Mar 2 '17 at 10:51
• Or just apply Newton's third law. Whenever some "sourceless" force appears, you need to find the equal and opposite force somewhere. In this case, it will be air resistance against the pipe as it's being compressed.
– SF.
Mar 2 '17 at 10:55
• Isn't that how steam powered vehicles worked? Mar 2 '17 at 12:22
• @4LPH4NUM3R1C No. Steam power involves dumpin high-pressure gas into a volume with a moveable wall (piston head) and forcing the volume to expand, with concomitent loss of pressure and temperature of the gas. Mar 2 '17 at 14:32